Field of the Invention
This disclosure relates to methods, systems, and apparatus of driving a motor, and more particularly, to methods for accurately generating current output commands when operating in a field-weakening region of the motor based on a reduced number of look-up tables.
Description of the Related Art
Electric motor control systems often involve the generation and storage of a plurality of data points for given input signals and corresponding output commands or values to select based on the input signals. In some embodiments, the data points may be stored in a data store, which may be conceptually formed as one or more lookup tables each corresponding to a particular DC input voltage (such as from a battery) to an inverter that generates 3-phase AC voltage inputs to the motor. In some implementations, a lookup table may exist for given increments of DC inverter input voltage through the voltage range of the DC inverter input that may be encountered during use of the motor. Each lookup table may be populated with a two-dimensional array of output current commands that control the inverter. Along one axis of each lookup table is a torque command value (e.g. a desired torque designated by the user of the motor), while along the other axis of each lookup table is a motor speed input (e.g. the rotation rate of the motor that exists at the time the torque command is received). Accordingly, the output current commands may correspond to a necessary command current in order to produce the torque command given a particular existing motor speed input and DC inverter input voltage.
The output current commands populating each lookup table may correspond to the particular values of voltage input, motor speed input, and torque command input. For example, at 50V, a motor speed input of 5000 RPM and a torque command input of 100 Nm may produce currents P1, whereas if the inverter input is 100V, the same motor speed and torque command inputs may produce currents P2, where currents P1 and P2 are different. The more lookup tables that exist (e.g., the more voltages that have associated lookup tables), the more accurate or granular the output current commands can be. If the DC inverter input voltage is a voltage that is between two voltages for which lookup tables are available, a linear interpolation process may be used to produce a current command output that is appropriately also between two lookup table entries in the higher and lower DC input voltage tables adjacent to the actual DC inverter input voltage.
However, this linear interpolation process may not be possible to accurately perform given the lookup tables in memory at high motor speeds and/or high torque command input. Additionally, creating these lookup tables takes substantial time and effort as they are based on experimental measurements made during motor testing.